Kurdish-led Syrian forces were closing in on the city of Minbij, the centre of the Islamic State extremist group's last stretch of territory along the Turkish border, a monitoring group said on Sunday.
The Kurdish-led Democratic Forces of Syria (DFS) were within 5 kilometres of the city's southern outskirts after six days of fighting, the Syrian Observatory for Human Rights reported.
The Hawar news agency, which is close to Kurdish forces in the DFS, reported that the commander of one of the alliance's components, the Northern Sun Brigades, died Sunday from injuries sustained during fighting east of Minbij two days earlier.
The commander, known by his nom de guerre Abu Layla, was the most senior DFS leader to have been killed in fighting with Islamic State since the alliance was formed in October, according to the observatory.
The monitoring group said he had also been involved in the defence of the border town of Kobane from Islamic State in 2014-2015 - a lengthy battle that drew worldwide attention to the Kurdish forces' resistance against the jihadists and ended up winning them US air support.
The DFS have also freed six women and 15 children from Iraq's Yezidi minority who were being held as slaves by Islamic State jihadists in the village of Khirbet al-Rus, the observatory said.
The jihadist organization enslaved or killed thousands of Yezidis after capturing the Sinjar region of Iraq, one of the religious minority's main homes, in June 2014.
The Syrian Kurdish YPG militia, the main component of the SDF, then mounted a cross-border operation to rescue thousands more who had fled to the barren heights of Mount Sinjar.
Islamic State views the Yezidis, whose ancient religion is linked to Zoroastrianism, as infidels. Yezidi men who were captured were reportedly killed or required to convert to Islam, while the group boasted of distributing Yezidi women and girls as concubines to its fighters.
Sinjar was recaptured by Kurdish forces in November last year. Several mass graves, thought to hold the remains of slain Yezidi civilians, have been found there since.